Aluminum borate whisker has a small coefficient of thermal expansion and is known to impart high mechanical strength to an epoxy resin. For example, JP-A-9-136372, JP-A-2000-239438, and JP-A-2000-345103 propose adding aluminum borate whisker to an epoxy resin composition. It has been considered that aluminum borate whisker having a diameter less than 0.3 μm is unfavorable for these applications in terms of dispersibility in a resin and applicability onto a substrate and that one having a diameter of about 0.3 to 3 μm is preferred as stated in JP-A-2000-345103.
If compounded with a large amount of aluminum borate whisker, however, an epoxy resin composition gains in viscosity and has difficulty being applied uniformly or provides a cured product with impaired mechanical properties, such as brittleness. Accordingly, aluminum borate whisker has been allowed to be added in an amount of, at the most, about 100 parts by weight per 100 parts by weight of an epoxy resin. It has therefore been demanded to find aluminum borate whisker that can provide an epoxy resin composition having superior physical properties even when incorporated in a large proportion.
When an epoxy resin composition containing aluminum borate whisker is applied to a substrate, the whisker is oriented in the direction of application. As a result, the cured epoxy resin composition exhibits different mechanical properties between in the direction of application and in the perpendicular direction thereto, which is disadvantageous for use in electronic circuit wiring boards. Therefore, it has been demanded to reduce such anisotropy of mechanical properties.